A mammalian mitochondrial (mt) genetic system based on resistance to chloramphenicol (CAP) has been previously developed. CAP resistance was shown to be cytoplasmically inherited by removing the nucleus of resistant (R) cells and fusing the cytoplasmic fragments to sensitive (S) cells (cybrid formation). Subsequent studies have demonstrated that CAP resistance is coded on the mtDNA, that there are mt incompatibility relationships between different cell lines, that mtDNA can be fixed in cells of different species, and that the nuclear coded mt genes are dispersed throughout the chromosomes. Recent evidence suggests that the RNA tumor viruses may use the mitochondria both for provirus synthesis and for virion synthesis and assembly. To test this association, I propose to subdivide virally-infected CAP-R cells and transfer the various fragments to uninfected CAP-S cells. Examination of co-transfer of the virus and CAP resistance should permit intracellular location of the viral genome. Since both the mitochondria and the RNA tumor viruses seem to regulate nuclear DNA (nDNA) synthesis as well as the genes necessary for their synthesis, it is possible that they might use the same or similar mechanisms for nuclear control. mt genetic studies will be extended with particular attention to their relationship to RNA tumor virus cellular regulation and host range. Studies on the number of mt compatibility types will be expanded and the mechanism of compatibility explored. These data will be related to parallel observations made on murine leukemia virus host cell specificity. Further studies on the arrangement of mt genes on the mtDNA and nDNA will employ inter-specific cybrids and mapping of additional mt mutants. These data, in conjunction with studies on the mechanisms of mt control of the nucleus, may result in further insight into tumor virus regulation of the cell. Finally, the effects of SV40 transformation on the mitochondrial phenotype of various human cells is being examined. These experiments should provide data on the role of the mitochondria in maintenance of the transformed state and the relationship of the mitochondria to tumor virus infection.